This invention provides a source of x-ray flux in which x-rays are produced by e-beams impacting the inner walls of holes or channels formed in a metal anode such that most of the electrons reaching the channel impact an upper portion of said channel. A small portion of the electrons will produce x-rays from this primary impact but most of them will be scattered, mostly in the forward direction of the e-beam trajectory, with the scattered electrons again impacting the walls of the channel and either generating x-rays or scattering, the scattered electrons then repeating the process until most of the electron beam has generated x-rays. A small portion of the beam will not generate x-rays at the channel walls through either primary or secondary (scattered) impact. This portion can impact a thin film of metal disposed across the diameter of the end of the channel, where it will either generate more x-rays or be drained away. The x-rays generated at the channel walls, and those few generated at the exit of the channel exit the channel out an anode window provided at the end of the channel. This anode window may support the thin metal film at the end of the channel.
Since the anode surface which generates x-rays in this source is many times greater than the corresponding surface of either the reflective or a transmission anodes of prior art x-ray sources, which are power limited by the generation of heat from e-beam impact, the disclosed source can also accommodate much higher electron beam current and therefore generate much higher x-ray flux from a given x-ray spot size. The disclosed source has the further advantage of pre-collimation of the exiting x-ray flux by the shape of the channel walls. It has a yet further advantage of hardening the beam, since some of the lower energy x-rays generated at the walls will be absorbed by the walls and higher energy x-rays will exit the channel.
A single channel x-ray source with high conversion efficiency and high power can be made with the disclosed forward flux channel (FFC) x-ray source architecture. This single channel source can be advantageously used in many applications, especially those now served by microfocus x-ray tubes, which commonly use a transmission x-ray target. In another embodiment, an FFC array source can be made with multiple channels in a broad anode plate, each channel receiving an e-beam from a cathode in a cathode array provided opposite the anode plate across the vacuum space of the source. FFC array sources, in linear or X-Y arrays, may be made as flat panels, as curved arrays or in other formats. They may be advantageously used in many other applications, including stationary computed tomography (CT) systems, parallel x-ray beam imaging systems and as wide sources of parallel x-ray pencil beams in phase contrast imaging (PCI) systems, coded aperture imaging systems or dynamically addressed coded source systems. In a further embodiment, the channels may be formed as long slits in the anode, to provide a fan beam of high power x-ray flux.
There is a continuing need for x-ray sources with higher flux levels and power efficiency. Particularly in x-ray imaging systems, an increase in flux power translates directly to a decrease in image acquisition time, to the limit of the detector. In x-ray analytical systems, the speed and scope of the systems is often limited by the flux available from the x-ray source used.
Prior art x-ray tubes with an angled reflective anode target are limited in their power output and efficiency by the fact that when the e-beam hits the anode surface only a small part of it penetrates the target material to generate x-rays; nearly half of the e-beam is scattered off the target back towards the cathode and loses power to make x-rays. Transmission anode x-ray sources have a fundamental limitation in generating x-ray flux in that the target must be a thin metal film to allow transmission of x-rays generated by the voltages used in imaging systems, but this thin film is inherently limited in the amount of heat it can dissipate and the heat it can handle before it melts or peals off the glass, beryllium or other flux exit window on which it is formed. Transmission targets also emit flux in all directions out the source. If collimators are used after the source, they will further diminish the already faint level of x-ray flux.
There are also a number of emerging x-ray imaging modalities which need new x-ray sources. Stationary CT systems, in which x-ray spots are addressed electronically in x-ray sources with multiple x-ray pixel (xel) locations, are being developed as an alternative to conventional CT systems using a classical x-ray tube rotating around a mechanical gantry. Various sources for these systems have been described in the prior art. Medical imaging typically requires e-beam current densities on the anode spot of at least a few A/cm2 at tens of kV electron energies, which is more power than a thin film transmission sources can handle before melting or delaminating. Angled xel array sources, such as those taught by U.S. Pat. No. 6,850,595 and U.S. Pat. No. 7,082,182, can handle higher power loads, but still may suffer anode pitting. Use of an angled target limits these sources to linear 1D xel arrays. Flat reflective anode sources, such as that taught in U.S. Pat. No. 8,155,273 and US 2010/0189223, can provide x-y xel matrixes, but they too would benefit from having a larger surface area over which to distribute the e-beam power.
Imaging systems in which multiple parallel x-ray flux beams pass through an imaging subject to be detected by a corresponding array of x-ray detectors, or an array of areas on a single x-ray detector, would have a number of advantages. More flux power could be generated by the use of multiple anode emission spots, since it is the instantaneous heat load on the anode which is most responsible for pitting or anode overheating. The use of multiple, limited-angle x-ray flux beamlets would also substantially reduce the amount of x-ray scatter in the subject, allowing a reduction in the radiation dose delivered to the subject. The increase in dose now commonly used to account for scatter in the subject, known as the bucky factor, could be cut reduced. With an x-ray source generating 77×77 or so of these x-ray beamlets, for examples, the bucky factor could be reduced by more than half in some imaging applications, such as breast imaging. Prior art sources, however, are not adapted to deliver multiple parallel x-ray beamlets. A flat panel source of the present invention, however, would be well adapted to such use and enable the development of new types of low dose imaging systems.
PCI is an emerging imaging modality which promises major improvements in dose reduction, improved sensitivity in low contrast applications such as breast imaging and high resolution. Prior art x-ray sources, however, are inadequate to make PCI useful for clinical and other large object imaging. Current PCI imaging systems rely on single pencil beams of x-ray flux, which do not cover a clinically meaningful area, or synchrotron radiation sources, which are large, expensive and not available in clinical settings. There has been research into the use of gratings to collimate and spread the flux from x-ray tubes over a wider area, but passing flux from a point source through a grating results in most of the flux from a point source being absorbed in the grating, resulting in unacceptably long image acquisition times. The source of the present invention can provide a highly parallel array of narrow or pencil beams, which can cover a wide area, and can be used with gratings and other PCI system techniques to make PCI available in clinical settings.
Coded source imaging is another new modality which promises high resolution, low noise and therefore low dose. It is possible to place a fixed coded aperture grating in front of an x-ray source and get a coded source but this too will have low flux power and long imaging times. The source of the present invention can be made with fine pitch xels to provide a coded source with high flux power. This source can also be dynamically addressed, for dynamic coded source imaging. This further enables coded source CT by shifting the coded source across a panel or array of panels.
There have been prior attempts to make a forward flux channel x-ray source. U.S. Pat. No. 4,675,890 teaches a rectilinear bore hole source with straight hole walls. Electrons at the high kV energies used in x-ray generation, however, are traveling at relativistic speeds and do not change course easily. Nearly all the electrons would pass straight through a straight channel and not generate x-rays. This prior art source teaches the use of magnets near the anode to deflect the beam into the channel walls, but this would be very hard to do by the time the electrons approach the anode and would require impractically large magnets. U.S. Pat. No. 6,993,115 also discloses forward flux channels in an x-ray anode, but this too has straight walls and relies on space charge spreading to direct some of the electrons into the channel walls. In reality, e-beams that are confined enough to make it from the cathode to the anode and into the channel will not suddenly start spreading due to space charge. Another source architecture, disclosed in U.S. Pat. No. 7,349,525, uses a flat anode disposed at a shallow angle on one side of a channel to receive the incoming electron beam. X-ray flux is then generated at a shallow angle and some of it passes through a collimating channel. While an improvement over prior sources, this source, by having the anode on only one side of the channel does not make use of the scattered portion of the electron beam and will therefore still have limited efficiency and power. It is also a large mechanical assembly, intended for use in a curved linear array of xels for a large stationary CT system and is not adapted for 2D parallel beam imaging, PCI or other of the imaging systems enables by the source of the present invention.
A need therefore exists for forward flux channel x-ray sources with improved power efficiency and power levels, adapted for use as single channel sources and for use in 2D arrays and dense arrays.